Femur component of a hip joint endoprosthesis

ABSTRACT

A femur component of a hip joint endoprosthesis which has a shaft ( 1 ) for anchoring in the medullary cavity of the femur. The shaft ( 1 ) has a distal section ( 9 ) and a proximal section ( 8 ), to which is connected a collar section ( 2 ) with a peg ( 3 ) for receiving an articular head, or with an articular head which is firmly attached to the collar section ( 2 ). The shaft ( 1 ) has a front surface ( 4 ), a rear surface ( 5 ), a lateral side ( 6 ), a mesial side ( 7 ) and a plane of symmetry ( 11 ), whereby longitudinal ribs ( 10 ) which stretch from proximal to distal are fitted on the front surface ( 4 ) and the rear surface ( 5 ) in the proximal section ( 8 ) of the shaft ( 1 ). The crests ( 12 ) of the longitudinal ribs ( 10 ) form an angle δ/2 of at least 1° to the plane of symmetry ( 11 ), and the envelope of the crests ( 123 ) of the longitudinal ribs ( 10 ) forms a double wedge-like or ellipsoid body, which tapers both toward the lateral side ( 6 ) and the mesial side ( 7 ).

FIELD OF THE INVENTION

This invention relates to a femur component of a hip-jointendoprosthesis with a shaft to be anchored in the femur's marrow cavityhaving a distal segment and a proximal segment joined by a neck with astub to receive a hinge head or with a hinge head firmly adjoining theneck and also including a posterior surface, a lateral side, a mesialside and a plane of symmetry, longitudinal ribs extending in theproximal-to-distal direction on the anterior surface and on theposterior surface in the proximal segment of the shaft.

BACKGROUND OF THE INVENTION

Femur components of this general type are known from the state of theart, but they incur various drawbacks.

As regards un-cemented femur prostheses for hip-joint replacement, theprimary stabilization of the femur shaft is implemented by frictionallyand geometrically locking onto the enclosing bone. The femur shaft isconfigured in such a way that loading it entails its being wedged intothe bony support. In particular, during the first loading phase whereinsome seating shifts of the femur shaft are likely, a suitableconfiguration must assure reliable primary affixation. In the event of aseating shift, new stabilization must be assured by suitablereconfiguration. In the absence of adequate primary stability, loadingwill entail repeated shifts at the boundary surface between femur shaftand bone, preventing reliable implant bodily incorporation. On the otherhand, if the primary anchoring is reliable, the implant can be enclosedby the bone tissue during the healing process and offer good long-termprospects.

Preferably, the primary affixation is in the upper portion of theprosthesis shaft enclosed by the spongy bone. A large support surfacecan be achieved in the big bone volume present therein. Seenbiomechanically and clinically, it has been advantageous to apply theforce through this region.

Illustratively, a femur shaft is on the market wherein the proximalshaft portion intended to be anchored in the spongy bone structurecontinuously tapers conically in the lateral-to-mesial direction inorder to secure renewed, automatic clamping in the event the bone yieldsin the mesial direction. The region of the trochanter major with theanchoring space, however, does not have a cross-sectionally triangularor trapezoidal shape, but rather an oval one. Accordingly, this knownfemur shaft suffers from the drawback that the laterally much enlargedproximal shaft portion may crack the bone. In addition, this known femurshaft comprises solid ribs which when displacing bone volume raises thepressure and may further contribute to the cracking effect.

A longitudinal section of the proximal femur with an inserted femurshaft shows that the spongy substance is not sharply delimited to thetrochanter region but instead partly continues as far as the zone of thediaphyseal bone tube. However, as much as possible of this bonestructure should be used to transmit the load. But the ribs located inone position of the known femur shaft do not optimally meet thisrequirement because of the little differentiated configuration. Thepoint of contact and the elongation of the ribs at the shaft should bedesigned in such manner that as much as possible of the spongy volume ofthe proximal femur is used for anchoring.

SUMMARY OF THE INVENTION

An object of the invention is to provide a femur component of ahip-joint endoprosthesis optimally corresponding to the spongyarchitecture in the proximal femur part and entailing cementless,primary shaft anchoring in the femur in the most stable possible mannerto secure thereby good likelihood of bone healing.

The double-wedge or ellipsoidal shape of the proximal shaft segmentoffers the advantage that the prostheses shaft can wedge itself bothlaterally and mesially in the event of a seating shift. The ovalenvelope curve of the ribs matching the cross-section of the proximalfemur minimizes the danger of cracking the proximal femur due to directpressure on the hard cortical bone.

In a preferred further development of the invention, wherein the ribsare cross-sectionally triangular, these ribs easily penetrate the spongybone volume and, as a result, the pressure is reduced during theinsertion procedure. Because, preferably, the triangular ribs extendconically, additional wedging is achieved that is lacking in rectangularribs such as are used in the state of the art.

The straightness of the shaft together with the increasing height in theproximal direction of the ribs extending in the direction of the shaftaxis allows secure positioning of the femur shaft and knocking it intoplace with guidance by the self-cutting ribs. If, on the other hand, theribs are partly or all mounted at an angle to the shaft axis, no seatenclosing the ribs can be realized when installing the femur shaft.Because the rib projection varies along the shaft, the stress on thespongy volume is more homogeneous than in known shafts with ribsbeginning at a given height which then continuously increases.

Another preferred development consists in that the combs of thelongitudinal ribs subtend an angle ½δ of at least 1°, preferably atleast 2° with the plane of symmetry. The individual combs of thelongitudinal ribs subtend different angles ½δ in the range of 3 to 8°with the plane of symmetry, preferably the longitudinal ribs situatedcloser to the lateral and the mesial side subtending a larger angle ½δthan those in between. Such a rib geometry functionally stimulates theenclosing bone, whereas such a stimulus is not achieved with the dullrib shape of the state of the art. This functional stimulus causes boneregeneration in the stressed zone with ensuing compaction and hence bonehealing. The blood supply to the regenerated bone can optimally form inthe troughs of this rib structure.

Appropriately, the anterior and posterior surfaces form a wedge taperingtoward the distal segment, the central plane being the plane ofsymmetry, the angle ε of the wedge being in the range of 0.5 to 3.0°,preferably within 1.0 and 2.0°. On account of this geometry, the wedgingeffect is continued also along the upper shaft zone. In case subsequentintervention is due on a solidly integrated shaft, the shaft is moreeasily knocked free if its geometry is conical in all directions, thatis, also proximally in the intra-rib zone, than if the geometry wereother than conical.

Seen in a section orthogonal to the plane of symmetry, the envelopecurve of the combs of the longitudinal ribs is approximately in the formof a kite quadrilateral of which the sides may approximately representstraight lines or arcs of circular or elliptical segments.

Relative to the mesial side, the kite quadrilateral should subtend aninside angle α larger than 10°, and preferably larger than 12°. Inaddition, the inside angle α should be less than 22°, preferably lessthan 20°.

Toward the lateral side, the kite quadrilateral should subtend an insideangle β larger than 8°, preferably larger than 9°. Moreover the insideangle β should be less than 45°, preferably less than 40°.

Appropriately, the longitudinal-rib combs are sharp and, seen in asection orthogonal to the plane of symmetry, are preferably triangular.Illustratively, the longitudinal ribs may assume the shape ofthree-sided pyramids of which the vertices point distally. Thelongitudinal-rib combs, however, may also be rounded and, seen in asection orthogonal to the plane of symmetry, preferably aresemi-circular. On the other hand, uniformly thick longitudinal ribs ofrectangular cross-section are to be avoided.

Preferably in a continuous manner, the width of the longitudinal ribsappropriately decreases from the proximal to the distal sides. This alsoapplies to the height of the longitudinal ribs which preferablycontinuously decrease in the proximal-to-distal direction.

Surprisingly especially good clinical results were observed when atleast one of the longitudinal ribs runs as far as the distal half of theshaft because this makes possible increased primary stability andbecause bone regeneration or bone transformation propagates proximallyfrom this anchoring zone in the form of osteo-conduction.

Further advantages may be achieved using embodiments wherein the shaftis without a collar and assumes a substantially rectangularcross-section as seen in a section orthogonal to the plane of symmetry.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention and its further developments are elucidated below withreference to several embodiments shown in the partly schematic figureswherein:

FIG. 1 is an elevation of the femur component of the invention seen fromthe anterior side and with two cross-sectional contours,

FIG. 2 is an elevation of the femur component of FIG. 2 seen from thelateral side,

FIG. 3 is a section of the femur component of FIG. 1 along line III—IIIand

FIG. 4 is a section similar to that of FIG. 3 with a modified envelopecurve of the longitudinal-rib combs.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The femur component of a hip-joint endoprosthesis shown in FIGS. 1through 3 essentially comprises a shaft 1 without a collar and with adistal segment 9 and a proximal segment 8 adjoined by a neck 2 with astub 3 to receive a conventional pivot head, or by a pivot head firmlyjoined to neck 2. Shaft 1 comprises an anterior surface 4, a posteriorsurface 5, a lateral side 6, a mesial side 7 and a plane of symmetry 11identical with the plane of FIG. 1. Shown in a section orthogonal to theplane of symmetry 11, the shaft is of a substantially rectangularcross-section 14.

Longitudinal ribs 10 are on anterior and posterior surfaces 4 and 5,respectively, in the proximal segment 8 of shaft 1 and extend from theproximal side to the distal side. Depending on their positions, combs 12of longitudinal ribs 10 subtend and an angle ½δ of 3° to 8° with planeof symmetry 11. Longitudinal ribs 10 near lateral side 6 and mesial side7 subtend a larger angle ½δ than longitudinal ribs 10 in between.Moreover, individual longitudinal ribs 10 are of different lengths,preferably those located toward lateral side 6 and mesial side 7 beingshorter than those in between. The line 17 connecting ends 18 of thelongitudinal ribs 10 merging into anterior and posterior surfaces 4, 5do not lie on a straight line but, instead, lie on a parabolic orellipsoidal curve.

The envelope curves of combs 12 of longitudinal ribs 10 subtend a doublewedge or an ellipsoidal body tapering both in the direction of lateralside 6 and in the direction of mesial sides 6, 7.

Furthermore, anterior surface 4 together with posterior surface 5 formsa wedge 4, 5 tapering toward distal segment 9, the plane of symmetry 11being the center plane, and the wedge angle ε of wedge 4, 5 being 0.5°.

As shown in FIG. 3, when seen in a section orthogonal to the plane ofsymmetry 11, the envelope curve of combs 12 of longitudinal ribs 10 forma kite quadrilateral 13, the quadrilateral's short sides pointinglaterally and its long sides pointing mesially.

The inside angle α of kite quadrilateral 13 is 12° to 20° toward themesial side 7 and its inside angle β toward lateral side 6 is from 9° to44°.

Combs 12 of longitudinal ribs 10 are sharp and, when seen in a sectionorthogonal to the plane of symmetry 11, each of their contours istriangular. The width and height of longitudinal ribs 10 decreasecontinuously in the proximal-to-distal direction. Accordingly,longitudinal ribs 12 form three-sided pyramids, the vertex of thepyramid pointing distally. Therefore, the troughs between individuallongitudinal ribs 10 narrow from distal segment 9 to proximal segment 8.

As shown in FIG. 1, one of longitudinal ribs 12, namely the center one,extends as far as the distal half of shaft 1 and thereby enhances theprimary stability of the implanted shaft.

The envelope curve of combs 12 of the longitudinal ribs 10, shown as akite quadrilateral 13 in FIG. 3, also may comprise slightly outwardbulging, for instance arcuate, envelope curves 15, as shown in FIG. 4.In this embodiment angles α and β relate to the inside angles of thekite quadrilateral formed by tangents 16 to the convex envelope curves.

What is claimed is:
 1. A femur component for a hip joint endoprosthesiscomprising: a shaft (1) to be anchored in a marrow cavity of a femur,said shaft having a distal segment (9) and a proximal segment (8), saidshaft further comprising an anterior surface (4), a posterior surface(5), a lateral side (6), a mesial side (7) and a plane of symmetry (11);a neck (2) attached to said proximal segment (8), said neck beingadapted to receive a pivot head; and a plurality of longitudinal ribs(10) extending in the proximal to distal direction on said anteriorsurface (4) and on said posterior surface (5) on said proximal segment(8) of said shaft (1), outwardly facing edges of said ribs comprising acomb structure, said edges lying in a double wedge or ellipsoidalsurface tapering in the direction of both said lateral side (6) and saidmesial side, wherein a width of each longitudinal rib decreasescontinuously in the proximal to distal direction, and a height of eachlongitudinal rib decreases continuously in the proximal to distaldirection.
 2. A femur component according to claim 1 wherein said edgesof said comb subtend an angle ½δ of at least 1° with said plane ofsymmetry (11).
 3. A femur component according to claim 2 wherein saidedges of said comb subtend an angle ½δ of at least 2° with said plane ofsymmetry (11).
 4. A femur component according to claim 2 wherein saidedges of said comb subtend an angle ½δ in the range of 3° to 8° withsaid plane of symmetry (11), said longitudinal ribs (10) situatednearest said lateral side (6) and said mesial side (7) subtending alarger angle than said ribs in between.
 5. A femur component accordingto claim 1 wherein said anterior surface (4) and said posterior surface(5) form a wedge tapering toward said distal segment (9) with said placeof symmetry (11) forming a center plane of said wedge, said surfaces ofsaid wedge subtending an angle e in the range of 0.5° to 3.0°.
 6. Afemur component according to claim 5 wherein said angle ε is in therange of 1.0° to 2.0°.
 7. A femur component according to claim 1 whereinsaid longitudinal ribs differ from each other in length with ribsnearest said lateral side (6) and said mesial side (7) being shorterthan ribs intermediately located.
 8. A femur component according toclaim 1 wherein said edges of said comb structure are sharp and whereinsaid ribs, as seen in a section orthogonal to said plane of symmetry,are triangular.
 9. A femur component according to claim 1 wherein saidedges of said comb structure are rounded and wherein said ribs, as seenin a section orthogonal to said plane of symmetry, are semi-circular.10. A femur component according to claim 1 wherein at least one of saidlongitudinal ribs extends distally to a lengthwise midpoint of saidshaft.
 11. A femur component according to claim 1 wherein said outwardlyfacing edges of a plurality of said longitudinal ribs on said anteriorand posterior sides lie in planes intersecting near said mesial side (7)to form an inside angle larger than 10°.
 12. A femur component accordingto claim 1 wherein said outwardly facing edges of a plurality of saidlongitudinal ribs on said anterior and posterior sides lie in planesintersecting near said mesial side (7) to form an inside angle largerthan 12°.
 13. A femur component according to claim 12 wherein saidinside angle is less than 22°.
 14. A femur component according to claim12 wherein said inside angle is less than 20°.
 15. A femur componentaccording to claim 1 wherein said outwardly facing edges of a pluralityof said longitudinal ribs on said anterior and posterior sides lie inplanes intersecting near said lateral side (7) to form an inside anglelarger than 8°.
 16. A femur component according to claim 15 wherein saidinside angle is larger than 9°.
 17. A femur component according to claim15 wherein said inside angle is smaller than 45°.
 18. A femur componentaccording to claim 15 wherein said inside angle is smaller than 40°. 19.A femur component according to claim 1 wherein said shaft has no collar.20. A femur component according to claim 1 wherein said shaft (1) issubstantially rectangular in section orthogonal to said plane ofsymmetry.
 21. A femur component according to claim 1 wherein saidlongitudinal ribs are in the form of three-sided pyramids with verticespointing distally.
 22. A femur component according to claim 1 whereinsaid outwardly facing edges of a plurality of said longitudinal ribs onsaid anterior and posterior sides lie in an envelope curve comprising akite quadrilateral.
 23. A femur component according to claim 1 whereinsaid outwardly facing edges of a plurality of said longitudinal ribs onsaid anterior and posterior sides lie in an envelope curve comprising anellipsoid or a lenticular shape.
 24. A femur component according toclaim 1 wherein distal extremities of said longitudinal ribs on saidanterior and posterior surfaces lie on a curve which is parabolic orelliptic.
 25. A femur component according to claim 1 wherein troughsbetween said longitudinal ribs decrease in width from said distalsegment toward said proximal segment.